This invention relates to a playback color processing circuit for a video tape recorder, more particularly to a color processing circuit possessing good vertical resolution and phase stability.
Video tape recorders are widely employed professionally and in the home for recording video signals on magnetic tape and playing them back. A video signal comprises a chrominance signal containing color information and a luminance signal containing brightness information. To enable a video tape recorder to handle the wide range of frequencies in a video signal, the chrominance signal, which has a carrier frequency of 3.58 MHz, is down-shifted to a lower frequency when it is recorded on the tape and up-shifted to its original frequency when played back. These frequency conversions are performed in the color processing circuits of the video tape recorder.
The playback color processing circuit thus receives a down-shifted chrominance signal and generates a chrominance output signal which has been restored to its original frequency. The playback color processing circuit includes an automatic phase control loop that locks the chrominance output signal in phase with a reference signal. As part of this loop, the color processing circuit has filters that remove unwanted frequency components from the chrominance signal and cancel crosstalk arising from adjacent tracks on the tape. To cancel crosstalk, prior-art color processing circuits generally use a simple comb filter comprising a single 1H delay line. (The symbol 1H denotes one horizontal cycle of the video signal; that is, the time from one horizontal sync pulse to the next.)
A problem with the use of this simple type of comb filter is that it degrades the vertical resolution of the circuit. Specifically, when chrominance is present in one raster line but not the next, the comb filter generates a spurious output signal in the next line with half the amplitude of the preceding chrominance signal. On the screen, this causes the color to appear smeared down into the next line.
A proposed solution to this problem employs a comb filter comprising a pair of 1H delay lines and other components interconnected in such a way as not to generate spurious output. This comb filter solves the vertical color resolution problem. However, it introduces a new problem in that the double delay slows the response of the automatic phase control loop. This leads to phase instability, which causes irregularities in hue.